1. Field of the Invention
The present invention relates to the measurement of properties of fluids in a flow stream. In particular, the present invention relates to a device and method for measurement of a fluid velocity profile that, in conjunction with a pressure drop measurement, allows for characterization of rheological properties of the fluid.
2. Description of the Related Art
In hydraulic fracture well stimulation operations, fluids such as gelled water fluids and cross-linked gels may be pumped downhole to create and extend fractures and place proppants therein. Fracturing fluid gel primarily contains fresh water ( greater than 99.6 % liquid phase), formate brine, diesel, guar (0.2-0.4% w/w liquid), sand (0-33% vol/vol liquid), and crosslinker or breaker polymers (0.1-0.4% vol/vol liquid phase). The preparation of the fracture fluid may involve three stages.
The first stage is the preparation of liquid gel concentrate (LGC). Water and biocide are mixed. Guar powder and diesel are mixed. Two mixtures are pumped separately into a hydration reaction tank. Monitoring of viscosity, density, and flow rates are needed at the inflows to this tank.
The second stage is the hydration reaction of LGC preblend at appropriate pressure and temperature. A viscous fluid (often called clean or base gel) is produced. Monitoring of the viscosity and flow rate is also needed for the base gel stream out of the hydration tank.
The third stage is blending of the base gel with sand and crosslinking agents to form a viscous, sandy gel fluidxe2x80x94the final fracture fluid. This blending may be the most important of the upstream operations. Measurements of viscosity, density, and flow rate of the fracture fluid from this stage are highly preferred. The finished fracture gel is then pumped at pressure (5000-10,000 psi) into distribution pipes and delivered to the well.
In brief, the fracturing fluids generally are non-Newtonian, multiphase fluids containing solid particles, water, and oils. The performance of these fluids is greatly affected by their rheological characteristics. It is greatly desirable to monitor in real time the rheological properties of these fluids over a wide range of shear rates. It would be advantageous for the monitoring apparatus to be non-invasive, and useable on-line, i.e. without disturbing the process flow stream or pumping operations.
As taught by D. W. Baker, xe2x80x9cPulsed Ultrasonic Doppler Blood-Flow Sensingxe2x80x9d, IEEE Transactions on Sonics and Ultrasonics, Vol. SU-17, No. 3, July 1970, hereby incorporated by reference, ultrasonic signals may be used for non-invasive measurements of fluid velocities. Ultrasonic techniques can provide accurate and reliable measurements. It would be desirable to adapt such techniques to provide a method for fluid rheology characterization.
The problems outlined above are in large part solved by a method and apparatus using ultrasonic signals to measure rheological properties of a fluid flow such as, e.g., the consistency index K, the flow behavior index nxe2x80x2, the yield stress xcfx840, or other parameters of any given model for shear rate dependent viscosity xcex7. In one embodiment, the method includes: (a) transmitting an acoustic signal into the fluid flow; (b) receiving acoustic reflections from acoustic reflectors entrained in the fluid flow; (c) determining a Doppler shift of the acoustic reflections in a set of time windows corresponding to a set of desired sampling regions in the fluid flow; and (d) analyzing the Doppler shifts associated with the set of sampling regions to determine one or more rheological properties of the fluid flow. The frequency shift caused by motion of the fluid is proportional to the velocity of the fluid, and this allows the construction of a velocity profile of the fluid flow stream. The velocity profile can be normalized and xe2x80x9cmatchedxe2x80x9d to one of a family of velocity profile templates, and the rheological properties identified by the curve that matches best. Alternatively, the shear rate as a function of shear stress can be calculated from the measurements, and these values may be used to find each of the parameters directly.
In one embodiment, the apparatus includes a transmitter, a receiver, and an electronic module. The transmitter transmits an acoustic signal into the fluid flow. The receiver receives reflections of the acoustic signal from entrained acoustic reflection sources in the fluid flow. The electronic module is coupled to the transmitter and receiver, and is configured to provide a pulsed high frequency signal to the transmitter and, responsive to the signal from the receiver, to determine a velocity vs. position profile of the fluid flow.